As the world's supply of fossil fuels is further depleted, the need to harness the energy in the wind assumes ever increasing importance. Unfortunately, wide use of this clean, inexhaustible source of energy has not occured because of the high cost of wind energy conversion devices.
In order to hold size and cost of wind turbines, towers, and machinery within bounds, almost all wind energy conversion systems utilize a means to limit the effect of high winds on the device. The American farm windmill has a tail vane which, when triggered by wind speeds exceeding its maximum set point, turns 90 degrees to the turbine shaft in order to rotate the turbine out of the wind. The 4-arm Dutch windmill relies on manual furling of canvas sails to accomplish the same effect. These devices are simple and inexpensive, but all the energy in the high winds is wasted.
In the more sophisticated designs, various methods are used to regulate the output energy or torque or delivered power of the system to a substantially constant level for all wind speeds above a predetermined value, called "rated wind speed". This method allows economical design of the wind turbine, tower, and machinery while recovering a large portion of the energy available in the high speed winds.
Many systems have been developed to perform this important function. Unfortunately, those that perform well are very complex and expensive while the low-cost units are generally applicable only to very small wind energy conversion devices.